One of the most common medical tests or procedures performed on patients is associated with the analysis of the blood of the patient or the infusion of fluid into the patient. Usually the first step in such a test procedure involves obtaining a sample of blood by invasive techniques. Of course, to puncture the vein of the patient, the vein must first be located. The location of the vein is not particularly difficult if it can be visually seen or felt. To enhance the probability of visual sighting or feeling, an elastic strap is often tightly wound around the upper arm of the patient. This produces a differential in the pressure of the blood being conducted by the veins. The human body responds to such a pressure differential by enlarging the veins in an attempt to provide a conduction path of less resistance. The enlarging of the veins makes them more prominent and therefore increases the probability that one of the veins can be located by viewing or feeling the arm of the patient. Unfortunately, the procedure for enlarging the veins is not always successful. For instance, because the vein is generally dark in color, it is even more difficult to sight a vein in the arm of a person having a dark colored pigment in his skin. Other characteristics of the patient that make it particularly difficult to sight or feel a vein are associated with small children, obesity, and old age. These characteristics generally mean that the vein is significantly recessed from the skin and therefore particularly difficult to visually sight or feel.
Various techniques have thus been developed to aid in the identification of veins. One such technique relies upon the fact that the temperature of the skin in proximity to a vein is generally greater than the temperature of the remaining portions of the skin. To detect the higher temperatures of the skin adjacent to the vein, liquid crystal materials have been employed that undergo a color change at the desired temperature. To improve color contrast, the liquid crystals are commonly applied to and viewed against a black background that serves to absorb the transmitted light. U.S. Pat. No. 3,998,210 to Nosari, for example, describes the use of encapsulated liquid crystals in a laminated article that includes a black background for locating veins in the body. Still another technique for enhancing the color contrast is described in U.S. Pat. No. 4,175,543 to Suzuki, et al., which involves cooling the skin with a cold pack before or after application of microencapsulated liquid crystals to produce a greater temperature gradient between the skin surface directly over the vein and adjacent areas of the skin. This temperature gradient is said to provide a sharper delineation of the vein for identification. One problem with the conventional vein identification methods, however, is that the liquid crystals employed generally have a low color density, poor color selectivity and are expensive. Further, the methods involved are too complex in that they often involve multiple steps to be performed by the user, such as color contrast, cooling, and so forth.
As such, a need currently exists for a simple, efficient, and effective method for rapidly identifying the presence of a vein.